Direct sequence phase modulation by pseudo-random spread spectrum codes has often been considered to allow use of a network of simultaneous multiple access communications and navigation using data-modulated signals occupying a common frequency band. Such a network is often referred to as a direct sequence code division multiple access (DS-CDMA) system. In such a system, each signal's unique spread spectrum code is used to digitally phase modulate the carrier at a ("chip") rate that is a very large multiple of the data modulation rate. Since this process spreads the signal bandwidth, the ratio of these two rates is termed the bandwidth spreading ratio (SR).
In prior art DS-CDMA receivers, signal selectivity has been accomplished in each receiver channel by correlation of the composite received signal with a locally generated synchronized replica of the desired signal's code. The result of this code matched filtering process is a selective enhancement of one desired signal's power over that of each of the other signals by a factor equal to SR, thereby suppressing mutual interference.
One of the major limitations to the use of DS-CDMA systems for radio networks using the conventional CDMA receiver structure is termed the near/far problem. This problem arises when weak and strong cochannel CDMA signals are received simultaneously. When the ratio of their received signal powers approaches or exceeds SR, the selective enhancement of a weaker signal is insufficient to overcome the interference from the stronger signals, and acquisition and accurate demodulation of the weaker signal becomes impossible. Since large ratios of signal strength are ofter associated with large differences in propagation distance, this situation has been termed the near/far problem. The subject invention is aimed at overcoming the near/far problem.
The prior art of DS-CDMA systems has been summarized in a journal article by Schilling et al, entitled "Spread Spectrum Goes Commercial," appearing in the August 1990 issue of the IEEE Spectrum. In that publication the authors envision using a technique called adaptive power control to overcome the near/far problem. Their technique requires sensing the received power level of each CDMA signal, and commanding the transmitters of those signals that are too strong to set back their transmitted power levels as needed to allow reliable demodulation of the weaker signals. This technique requires an active return communication link from the receiver back to each active transmitter and the means to adjust the power of each CDMA transmitter. The subject invention uses a completely different approach to overcoming the near/far problem, one which is totally contained within the receiver itself so that no active return communication links to the transmitters are required and one that can operate with simpler constant power transmitters.
Accordingly, it is an object of the present invention to provide an improvement in DS-CDMA receiver apparatus.
It is another object of the present invention to provide a DS-CDMA receiver apparatus which includes improved signal selectivity by using correlations with each received signal's spread spectrum code, not only to selectively enhance each received signal but also to selectively suppress all the other DS-CDMA received signals, thereby overcoming the near/far problem.
It is another object of the present invention to provide a DS-CDMA receiver apparatus which includes means for estimating and reconstructing replicas of the waveforms of the DS-CDMA received signals with minimal time delay, and then subtracting these reconstructed waveform replicas from the receiver input so as to effect cancellation of their corresponding DS-CDMA received signals.
It is another object of the present invention to overcome the near/far problem without the requirement for a return communication link to the CDMA transmitters and without the requirement that the CDMA transmitters have means to adjust their output RF power level.